Incandescent lights have a longer life when operated at a lower voltage. A growing practice is to include a voltage reduction device such as a rectifier to reduce the root mean square voltage. A rectifier converts the alternating current commonly available to direct current, and lowers the root mean square operating voltage to approximately the peak voltage of the alternating current divided by the square root of two. One inexpensive method of rectifying the current is to include a diode in series with the electric lamp. The filament coil may then be redesigned for the lowered voltage to be more resistant to shock and to have a longer life. The result is increased lamp life.
U.S. Pat. No. 3,823,339 to Edmond H. Borneman et al. and U.S. Pat. No. 4,229,680 to Donald M. Berlin Jr. et al. show diode devices designed to be inserted in a lamp socket prior to the insertion of the lamp. The insert is then designed to generally convert the available alternating current to direct current for any subsequently inserted lamp. The reduced voltage increases lamp life, but also affects the quality of the light output, and in particular lowers the color temperature of the light. In summary, the lamps used with the insert are not redesigned to optimize the change in current and voltage.
Unfortunately, diodes can fail. One failure mode is due to large line surges which occasionally occur. One estimate is that a large line surge occurs about one thousandth of one percent (0.001%) of the lifetime of a lamp with a life of 3,000 hours. Line surge faults affect all lamps, regardless of type or mounting configuration. U.S. Pat. No. 3,930,183 to John D. Harnden, Jr teaches, the inclusion of a varistor material to protect a lamp filament against line surges. Varistor materials have resistances that decrease with increasing voltages, but there is no difference with respect to polarity. Varistors are then not useful in generally lowering the voltage.
A second failure mode is due to overheating. Many lamps are designed to achieve the maximum light output for a particular lifetime by maximizing the wattage for the filament. As a result, many lamps run fairly close to their temperature limits under normal conditions. Additional heat may lead to failure. A lamp enclosed in a small space may be operated for a sufficiently long time to heat the surrounding volume to above the critical temperature of the rectifier. Alternatively, the environment of the lamp may be heated by exterior heat sources, again to the point of stressing the lamp components beyond their critical limits.
The rectifiers currently used for rectification from alternating to direct current operate well up to about 225.degree. C. If the rectifier should fail, as for example in a closed in fixture, the voltage seen by the lamp rises from the rectified line level, of for example 84 volts, to the normal, unrectified line level, of for example 120 volts. The filament as a result runs hotter In some instances, the hotter running filament fails quickly In other instances, the filament may not fail for many hours at the higher wattage. As a result, heating increases and another portion of the lamp, or socket may fail instead of the filament. When lamp jacket, socket or other nearby elements fail, there is the possibility of containment failure or even fire. Known fire tests, called cheese cloth tests, have shown that a lamp with a rectifier that normally passes the test, may fail the cheese cloth test when the rectifier fails in a shorted mode causing a voltage increase and therefore a heat increase
There is a need to detect failure of a rectification device, and a corresponding need to respond to the failure of the rectification device. There is a need to provide a safety mechanism in a rectified current lamp. Electric lamps are relatively inexpensive, and mass produced. Accordingly, the solution to the safety problem needs to be both inexpensive and capable of mass production. Complex components and circuit systems are then inappropriate for the solutions sought. Furthermore, simplicity of manufacture is important to overall cost and reliability.